[0001] The present invention relates to improvements in the microwave heating of frozen
food meals. In particular, the invention relates to a container for use in re-thermalisation
of frozen food blocks in microwave oven. The invention is more directed to improvements
of large size frozen meals that usually require excessively long heating time.
[0002] The reduction of microwave reheating of large size frozen meals is a real concern
in food service or collective catering area. For individual portions or small size
frozen meals, the reconstitution in a domestic microwave oven, can be carried out
in a relatively short period of time, generally in the range of 2 to 6 minutes, depending
on the type of foods, on the size and lay-out of the various food component in the
tray, etc. However, for large size frozen meals, the microwave reconstitution has
proved to be excessively long, up to 30 minutes, which renders the use of microwave
oven less attractive.
[0003] Another problem in microwave re-heating of frozen products is related to the temperature
gradient which occurs in most of the known containers. Before the thawing, the frozen
product is almost transparent to microwaves which are absorbed at a very low rate,
or even not absorbed at all. In that situation, in a regular microwave transparent
container, the microwave energy is not properly absorbed by the frozen mass while
the interface region with the container concentrates a major proportion of energy.
This unevenly energy distribution is not equalised by convection heat transfer which
so causes excessive heating at the edges of the container whereas the core of the
frozen mass remains at very low temperature. The microwave heating pattern of a large
frozen dish is generally characterised by the presence of large cold spots in the
centre of the upper surface, by the very late thawing process of the inner parts of
the products and by the overheating of the edges and corners.
[0004] EP 348 156 to Hewitt relates to an improvement relating to microwave heating where
a microwave mode is generated from underneath the food product. The food product is
disposed in a transparent tray placed on a stand so that a predetermined elevation
is maintained between the bottom surface of the food product and the internal bottom
surface of the stand. The heating from underneath may occur by placing separated electrically
conductive plates at the bottom of the stand which is made of a microwave transparent
material, or by making apertures in the electrically conductive bottom of the supporting
stand. The purpose is so to have a majority of the microwave energy enters through
the undersurface of the container to maximise the bottom heating effect.
[0005] EP 185 488 to Sugisawa relates to a container, made of a material transparent to
microwave, heated by microwave oven in which a microwave reflecting strip partly covers
the region of the container where the upper surface of the material contacts the side
of the container. The main object is to prevent local over-heating of the food product.
However, the container brings no significant improvements to the reheating of frozen
food but simply proposes to solve a local problem of burning of the edges in a conventional
transparent container.
[0006] EP 471 969 to Payne relates to the use of a microwave suceptor sleeve for pizza and
the like onto which the food items are placed. The supporting base onto which the
suceptor is placed with the food product on it, may be elevated with respect to the
oven sole by the use of pre-cut legs; and one side of the box includes a microwave
reflecting sheet. The elevation of the base supporting the suceptor by the use of
pre-cut legs is dictated by the need to separate the suceptor from the microwave oven
sole in case there is no glass shelf in order to eliminate the risk of arcing.
[0007] US 5,310,980 to Beckett refers to the incorporation of metallic patches on a microwave
transparent tray in order to orient in the desired way the impinging microwave energy
beams towards parts of the product that do not heat-up appropriately.
[0008] EP 350 660 A2 to Jaeger relates to a suceptor sheet with the microwave transparent
packaging.
[0009] US 4,642,434 to Cox and al. relates to a microwave reflecting energy concentrating
spacer that includes in its lower part a microwave reflector separated from the food
base by a distance of about ¼ of a wave length, that is 3 cm as the free space wave
length at the microwave emitted frequency in the microwave oven (2.45 GHz) is about
12 cm.
[0010] EP 242 026 A2 to Swiontek discloses an assembly between a suceptor which is described
as a "microwave interactive layer" and the whole package.
[0011] US 4,656,325 to Keefer refers to "cold suceptors" consisting in placing metallic
patches disposed in a regular array on the cover of the pan containing the food product.
[0012] US 4,888,459 to Keefer also refers to "cold suceptors" with in addition and optimisation
of the thickness and the dielectric permitivity of the material constituting the non-reflecting
part.
[0013] US 5,270,502 to Brown and al. relates to a combination of a microwave interactive
layer that is in fact a suceptor and a supporting stand made of a microwave transparent
material.
[0014] WO 95 24 110 to Gics relates to an ovenable food package comprising a microwave suceptor
placed beneath the food base in order to induce the crispening of the food base.
[0015] US 4,496,815 to Jorgensen relates to a microwave browning utensil comprising a metallic
base with a ferrite layer which is a highly microwave absorbing material.
[0016] US 4,542,271 to Tanonis et al. relates to a microwave tray comprising absorbing material
placed beneath the bottom surface of the tray.
[0017] US 4,927,991 to Wendt et al. relates to a microwave oven package comprising a combination
of a grid and suceptors inside a microwave-transparent tray which behaves like a conventional
frying pan as it is heated by microwave radiation through the tray.
[0018] Other prior art documents on microwave packaging are US 4,994,638 and US 4,535,889.
[0019] There is still a need to propose a container for frozen food product that (1) properly
prevents the temperature gradients but promotes a uniform and efficient distribution
of heat within the product and, (2) accelerates the microwave reheating of frozen
food product, in particular, of large size frozen food block.
[0020] For that the present invention relates to a container comprising
(a) a tray comprising a bottom wall and
a continuous side wall attached to said bottom wall which extends upwardly from
the bottom wall, said bottom wall and side wall defining an interior cavity, and
(b) support means for providing support to a frozen food block, said support means
being capable to be positioned within the cavity to maintain the food block in an
elevated position with respect to the bottom wall so as to form a predetermined vertical
free space between the bottom wall of the tray and the frozen food while the frozen
food is heated, the support means being made of a substantially transparent material
to the microwave, wherein,
the bottom wall and at least part of the side wall comprise at least one continuous
shielding layer which horizontally and peripherally delimits the free space, underneath
the food block, so as to totally reflect the microwave beams toward the food block.
[0021] It has been surprisingly found out that by both elevating the product and providing
a laterally and horizontally free spacing reflecting microwaves under the food block
that a mode of total reflection of the reflected radiation was generated within the
food block. It has also been found that the energy reflected back by the container's
cavity to the microwave source, e.g., the energy that was not absorbed by the product
and then lost, was significantly reduced compared to the state-of-the-art containers.
Therefore, the structure as proposed by the container of the invention induces an
improved coupling between the food product and microwaves, and thus the rapid heating
of the product as most of the available microwave energy is absorbed by the food product
instead of being lost by reflection back to the generator.
[0022] In an embodiment, the continuous shielding layer extends upwardly along the side
wall at least beyond the region where the frozen food upper surface contacts the side
wall. Thus, the container's sides are properly shielded so as to reflect and to concentrate
a maximum amount of microwave energy within the cavity. This configuration also contributes
to a more homogeneous microwave energy distribution within the food block which is
so allowed to thaw and heat more rapidly without cold spots.
[0023] In an embodiment, the free space has a vertical length of at least 2 mm. Preferably,
the vertical length is comprised between 5 to 20 mm. If the elevation of the food
block is insufficient, the microwaves penetrating the product from the top surface
propagates until reaching the internal bottom surface, and then are reflected back.
However, the crossings by microwaves are done in conditions that leaves very small
chances for microwaves to be absorbed by the product, because of the inappropriate
angle of incidence of microwaves. As a matter of fact, about 80 % of the impinging
microwave energy have a normal incidence on the top surface of the product and hence
in the absence of a free space underneath the food base, most of the reflected energy
would propagate with a normal direction, which would lead to a very low amount of
microwave absorption by the food product. The elevation permits to produce a more
inclined incidence angle of the reflected beam which is no more normal so that the
microwave absorption can increase. The given range represents the optimum elevation
of the food product with respect to the continuous shielding layer at the bottom of
the container where most of the microwave energy remains within the product through
a multiple internal reflections which can so occur between the upper and the lower
surfaces of the food product. It has been surprisingly found that within that defined
range, the heating rate is increased by about 50 to 80% by the under-heating effect
and the multiple internal reflective pattern, before the thawing of the product, i.e.,
during the period the product is usually less incline to absorb energy, as previously
discussed.
[0024] In an embodiment, wherein the container comprises a lid, at least a portion of which
is adapted to form the support means after the container has been opened. Preferably,
the lid is capable to fit the tray in an up-side-down configuration so as to be capable
of receiving the food block at a predetermined elevation. This construction provides
a simple and economical appropriate solution by reducing the number of elements.
[0025] In another embodiment, the support means is collapsible. For example, it may comprise
a supple inflatable bag comprising a series of air cells defining interior channels.
The support means is so less cumbersome thus avoiding the need to oversize the container.
[0026] In an embodiment, the container comprises an assembly of interchangeable tray members,
a first microwave-transparent member in which the food block is positioned and a second
reflective-microwave member of larger section externally engaging the first member
in a closed configuration for closing the container, the first member being capable
to fit at a predetermined elevated depth within the second member when reversed in
a heating configuration. Such a construction is economical and of a convenient use
for the consumer. The container can also have compact over-all dimensions when stored.
The number of pieces is also advantageously reduced.
[0027] According to a still further aspect of the invention, there is provided a method
of heating food products in a more efficient and homogeneous manner comprising
positioning the food product in a container while leaving a sufficiently large uppersurface
of the container transparent to microwave energy to allow an amount of microwave energy
to enter the food product,
leaving a predetermined free space under the food product by elevating the food product
in the container,
providing a continuous shielding arrangement of at least a part of the container corresponding
to the external limits demarcating the free space so as to produce a total reflection
of the microwave beams toward the food block.
[0028] The advantages and specific features of this invention will become apparent from
the following detailed description, which taken in conjunction with the drawings,
discloses preferred embodiments of the present invention.
FIG. 1 shows a vertical medial cross-section of a container of the invention according
to a first embodiment,
FIG. 1A is an enlarged partial detailed view of FIG. 1,
FIG. 1B is an enlarged partial detailed view of FIG. 1 according to a variant of the
invention,
FIG. 2 is a diagrammatic top plan view according to FIG. 1,
FIG. 3 is a diagrammatic bottom view according to FIG. 1,
FIG. 4 is a cross sectional view similar to FIG. 1 but according to a second embodiment,
FIG. 4A is an enlarged partial detailed view of FIG. 4,
FIG. 5 is a top view of the tray of the embodiment of FIG. 4,
FIG. 6 is a cross sectional view similar to FIG. 1 but according to a third embodiment,
FIG. 7 is a top view of a container according to another variant,
FIG. 7A illustrates a partial cross-section view of a detail of FIG. 7,
FIG. 7B illustrates a partial cross-section view of a detail of FIG. 7 according to
another variant,
FIG. 8 is a diagrammatic view showing the microwave propagation according to the principle
of the invention,
FIG. 9 is a thermograph diagram of the heat distribution of a food product which has
been submitted to microwave radiations from above in a conventional microwave transparent
tray for 15 min,
FIG. 10 is a comparative thermograph diagram of the heat distribution of a food product
which has been submitted to microwave radiations from above in a container of the
invention for 15 min,
FIG. 11 is a thermograph diagram of the heat distribution of a food product which
has been submitted to microwave radiations from above in a conventional microwave
transparent tray for 20 min,
FIG. 12 is a thermograph diagram of the heat distribution of a food product which
has been submitted to microwave radiations from above in a container of the invention
for 20 min,
FIG. 13 is a diagrammatic concept of a container assembly of the invention according
to another variant,
FIG. 14 is a diagrammatic view of a variant of the container of FIG. 13,
FIG. 15 is a diagrammatic view of container according to another variant,
FIG. 16 shows a support element of the embodiment of FIG. 15,
FIG. 17 illustrate another concept of container in a closed configuration according
to another variant of the invention,
FIG. 18 shows the container of FIG. 17 positioned in a configuration ready for heating
of the food block contained therein in a microwave oven.
[0029] A container of the invention is indicated by the numeral reference 10 in FIG. 1,
for example. The container 10 comprises a tray 20, which has a bottom wall 21, and
a side or peripheral wall 22 extending from the bottom wall. The conjunction of the
bottom wall and side wall defines an interior cavity 23 which can be, optionally closed
by a lid 4, as shown in dotted lines. In the context of the invention, the lid is
preferably removable before being inserted in the microwave oven. In case, the lid
is non-removable, the lid will be made of a suitable transparent material to microwaves,
e.g., a plastic, cellulosic, ceramic or fibreglass material. It is important to note
that the container of the invention needs to offer a relatively wide microwave transparent
uppersurface or window for being properly fed by the microwave energy.
[0030] Within the cavity 23 of the tray is positioned a support means 3 which includes preferably,
in that specific configuration, a plate-like portion 30 which supports the load of
a frozen food block 5. As shown in FIG. 1 and 3, the plate-like portion is spaced
from the interior surface 210 of the bottom wall 21 by means of a series of spacing
members 31, 31a, 31b, 31c, 31d, 31e. The spacing members are preferably evenly distributed
under the plate-like portion 30 so as to avoid any unbalanced position and ensure
a relatively constant free space 6 between the food block and the bottom wall. The
spacing members are preferably attached to the plate-like portion. More preferably,
they are made unitary with the plate-like portion.
[0031] The support means are made of suitable microwave transparent materials having also
sufficient rigid characteristics for properly supporting the food block. Plastic,
cardboard, ceramic, fibreglass, glass or any suitable combinations thereof can be
used. Metallic materials are excluded, as the beams would not reach the free space
6 but would be reflected toward the food block at wrong incidence angle.
[0032] According to an important aspect of the invention, the tray 20 comprises a continuous
shielding layer which permits the reflection of the microwave beams toward the food
block with a reduced amount of non-absorbed microwave energy. In the present context,
"continuous" means that the layer is free of any apertures which could allow the beams
to escape or the beams to enter from underneath of the container and consequently
modify the heating pattern in an non-suitable way. In FIG. 1 and 1A, the tray is entirely
made of a monolithic material reflecting microwave radiation. The tray is so preferably
made of a one-piece material for cost reasons and ease of construction. By reflecting
material, it must be understood any material that reflects at least 90 % of the microwave
energy. Preferably, the material is an aluminium or an aluminium alloy. This continuous
integral shielding arrangement permits to provide an intense and total reflection
both laterally and underneath of the food frozen product with no risks of overheating
the edges of the food product as in a conventional microwave tray. As shown by FIG.
1A, a free space 6 is externally and continuously delimited both horizontally by the
reflective interior bottom surface 210 and laterally by the reflective interior lateral
surface 220 of the tray. This important aspect has proved to confer a surprising modification
of the reheating regime, characterised by a much more uniform heat distribution within
the food product with lowered temperature gradients. Therefore, contrary to the numerous
prior art on "suceptor patches", such as EP 348 156 for example, the present invention
confines the microwave fields into the product in the tray by shielding the bottom
of the tray and at least part of the edges of the tray. The presence of apertures
in the tray would completely destroy the microwave pattern into the food product and
thus reduce the observed substantial increase of the microwave energy absorbed by
the food product.
[0033] FIG. 1B shows an alternative of the invention in which the shielding layer is a separate
layer 70 coated onto a rigid frame 71 of the tray. Therefore, the tray can be made
of a multi-layered arrangement or laminate of combinations of shielding and transparent
layers provided at least one of the layer is a continuous layer which is impervious
to microwave radiation. Layer 70 can be made of a metallic layer, preferably of an
aluminium or aluminium alloy. In the illustrated example, layer 70 is the internal
layer and the rigid layer 71 is the external layer. However, layer 70 could also be
positioned as the outermost layer of the tray or as an intermediate layer between
two transparent layers of the tray. The reflective layer can contemplate a wide range
of stiffness from the very flexible to the very rigid.
[0034] More generally, in the present description, the reference to a free space 6 has to
be understood as the space vertically defined by the distance or vertical length
L provided between the surface of the continuous non-transparent shielding layer of
the bottom wall 21 and the bottom surface 50 of the frozen food block. Generally,
the bottom surface 50 of the food block being adjacent to the upper surface 32 of
the plate-like portion, the free space can be considered as the length
L between the non-transparent shielding layer and the upper surface 32 of the plate-like
portion 30. The free space 6 is also horizontally demarcated by the side wall of the
tray, more particularly, by the shielding layer of the side wall. Both the bottom
surface and the side wall of the tray participates to the continuous external demarcation
of the free space underneath the food product in the sense that no microwave radiation
can enter or leave the free space both in horizontal and downward directions.
[0035] Turning now to FIG. 8 which shows a pattern of interactions of microwave with the
food product in the context of the invention. As depicted in that figure, the microwave
initial radiations 80 are fed from above of the container by a microwave source such
as an assembly comprising a magnetron and a wave guide (not represented). As the radiation
beams encounter the food product, a diffraction of the beams occurs leading to a diffracted
radiation component 81 within the food product. The beams enters the shielded spacing
chamber located below the food product where the diffracted radiation component is
reflected under a predetermined incidence angle so producing an entrapment effect
between the upper surface and the lower surface of the food product until at least
60 to 70 % of the microwave radiation is absorbed. As the microwave fields propagate
by crossing in a relatively symmetric way into the product before escaping it, the
angle of incidence of the refracted beams is optimised in order to extend the electrical
path within the food product and thus to increase the microwave absorption. It is
possible to obtain substantially complete internal reflected radiations at the upper
product-air interface. Such situation depends on the thickness of the free space underneath
the food product but also may depend on the thickness of the product and dielectric
properties of the food product.
[0036] FIG. 4, 4A and 5 show an alternative to the previous embodiment in which the spacing
member is part of the tray. In that particular case, the spacing member forms a peripheral
shoulder 24 onto which a plate-like portion 30 is positioned. The shoulder may be
either continuous or made of discrete shoulder portions, as well, provided the plate-like
portion is in a static arrangement over the free space 6. Additional spacing members
could be added to prevent the plate-like portion to flex in the middle of the tray
which otherwise would make the length of the free space non-constant and consequently,
would modify the heating regime in the middle of the food block compared to the edges
of the food block.
[0037] FIG. 6 illustrates another alternative which was only used for experimenting the
present invention. The spacing means are, in that case, a plurality of transparent-material
marbles 33 distributed on the surface 210 of the bottom wall of the tray. The marbles
directly contacts the bottom surface 50 of the food block. Suitable glass or plastic
marbles can be used. This embodiment is only described as an experimental alternative
but would probably not be suited for a convenient commercial use as the food, if partly
flowable, would mix with the marbles after thawing.
[0038] In the present invention, the container may comprise a tray of conventional or original
shape, i.e.; a rectangular, square, round, or polygonal sided tray is suited. Trays
having a high reflecting capacity as the one of the invention, the corners of angled-side
tray may require a higher concentration of microwave radiation so as to allow browning
and crisping in that regions. For that, the side walls which comprises a number of
angled portions 221, 222, 223, 224, for example four portions in the case of FIG.
7, can advantageously be covered in their immediate vicinity by upper microwave opaque
layers 41, 42, 43, 44. Preferably, the opaque layers would form triangular-shaped
trapping regions. In an embodiment, the lid 4 would have corners integrally formed
by the opaque layers 41 to 44 as shown in FIG. 7 and 7A. The rest of the lid would
be made of a transparent layer. The lid would so remain in place during thawing and
heating in the microwave oven. FIG. 7B illustrates an alternative in which the opaque
layers are additional layers secured in adjacent configuration to a transparent lid
4.
[0039] Microwave reheating trials have been performed according to the embodiment of FIG.
6, on frozen lasagna products. Glass beads having 10 mm were tested in order to simulate
height elevation of the product with respect to the bottom surface of an aluminium
tray. In addition, the four corners of the aluminium tray were covered with aluminium
patches of triangular shapes having about 6.5 cm side length along the edges of the
tray. The reason for the patches was to boost somehow the reheating regime of all
the lasagna components in the corners including béchamel sauce. The frozen lasagna
weighted about 1 kg. The tray had a rectangular configuration with the following dimensions:
23 cm X 17 cm X 3.5 cm. The reheating trials were carried out using a Panasonic Genius
NN-6858 side-fed microwave oven delivering a power output of 720 Watts, equipped with
a turntable.
[0040] For comparison purpose, FIG. 9 shows a thermogram of lasagna reheated in a conventional
microwave transparent tray for 15 min in the Panasonic microwave oven. The thermogram
is performed using an infrared camera for the overall temperature distribution of
the upper surface of the product. FIG. 9 shows very contrasted temperature gradients
with very low temperatures in the middle of the lasagna product (1A) and hotter regions
in the vicinity of the periphery of the product (1D). In-between, the temperatures
vary in a substantially gradual relationship. Therefore, after 15 min, the lasagna
product is still not at the right temperature in core while the edges are starting
to heat.
[0041] FIG. 10 shows a thermogram using the container of the invention with 10-mm elevation
of the free space. The large cold spot has disappeared completely replaced by a substantially
uniform temperature distribution on the top surface of the product. A large warm zone
2A at about 60°C covers a major part of the upper surface of the food block after
15 minutes in the microwave oven.
[0042] For 20 min heating in the Panasonic oven as illustrated by FIG. 11, the thermogram
of the conventional microwave-transparent container still shows a high temperature
gradient with a centred cold spot 3A at only about 15°C. On the contrary, the thermogram
of FIG. 12 shows a large hot spot 4A at a temperature of about 81.5°C in the centre
of the surface of the lasagna as reheated in the optimally designed container of the
invention. In fact, after 12 min heating, the upper layer of the lasagna starts to
expand and to form some "waving". After 15 min heating the upper parts of these "waves"
start burning. Visually, it is very appealing to have such browning and even burning
on the top surface of the lasagna.
[0043] Results for the pertinent microwave heating parameters for the heating of lasagna
for 15 min in the Panasonic according to the designed tray with glass beads having
a diameter of 5, 7, 8, 10 and 12 mm are listed along with an aluminium tray with no
elevation (equivalent to a direct contact of the food block with the bottom of the
tray) in the following table. For comparison purpose, a lasagna in plastic tray reheated
for 30 minutes corresponding to a substantially complete microwave heating has also
been measured. The results are given in Table 1.
TABLE 1
Comparative |
ΔTm /Δt (°c/min) |
σ |
TI (°C) |
◆ Lasagna in Plastic Tray |
3.03 |
8.3 |
41.5 |
◆ Aluminium Tray. No elevation |
2.87 |
3.6 |
-2.5 |
Invention |
ΔTm /Δt (°C/min) |
σ |
TI (°C) |
◆ Elevation 5mm |
4.27 |
2.6 |
13.5 |
◆ Elevation 7mm |
4.88 |
2.8 |
28.5 |
◆ Elevation 8mm |
5.22 |
2.5 |
44.8 |
◆ Elevation 10mm |
5.65 |
2.2 |
55.2 |
◆ Elevation 12mm |
5.35 |
2.9 |
50.8 |
[0044] The apparent mean heating rate is termed "ARH" and formulated by ΔTm/Δt where Tm=
Tm-Ti with Tm: the mean temperature on the top surface as deduced from the thermogram
and Ti: the initial temperature which is -20°C, and Δt is the heating time (30 min.
for the plastic tray and 15 min. for the aluminium trays).
[0045] σ is the calculation of standard deviation of the upper side temperature distribution
in the thermograms. The lower the value of σ, the more uniform the temperature on
the top surface of the product.
[0046] TI is the lowest temperature of the product measured after 15-min heating time using
fibre-optic probes, which are, located about 1.5 cm beneath the centre of the coldest
areas detected on the thermogram.
[0047] It is flagrant that the use of an aluminium tray with or without the elevation leads
to a uniform heating pattern on the top of the product, as indicated by the σ values
which are reduced by a factor of about 4 (8.2 to 2.2). The ARH (Apparent Heating Rate)
which is decreased by the use of an aluminium tray as compared to a plastic tray,
shows however a steep increase with increasing elevation of the product with respect
to the bottom surface of the aluminium tray. The elevation by about 10mm seems to
be the optimal elevation for this product having a thickness of about 28mm. For a
higher elevation, the trend for the improvement of ARH is slightly reversed.
[0048] For all the tests performed using the aluminium tray instead of a plastic tray, the
temperature pattern obtained is so by far more uniform. However, when there is no
elevation of the lasagna product in the aluminium tray, or when the elevation is far
away from the optimal one, the deepest parts of the lasagna remain frozen and for
extended reheating time, they start to thaw slowly. Close to the optimal elevation,
the interior of the lasagna starts to thaw at the beginning of the microwave reheating
process and the overall microwave heating rate is drastically improved.
[0049] In the optimal conditions consisting in the use of a container of the invention with
an elevation of about 10mm with respect to the tray bottom surface, the complete reheating
of 1Kg of lasagna having a thickness of about 28mm, may be achieved in 15 to 16 minutes.
This corresponds to a reduction of about 50 to 54% in the microwave reconstitution
time of the lasagna.
[0050] The invention is particularly adapted for reheating of large size containers of at
least 1 litre. However, smaller container such as those adapted for single portion
frozen meals for reheating in domestic ovens could also benefit from the invention.
[0051] FIG. 13 shows another solution of the container 10 of the invention in which it is
constituted of a tray 20 and a lid 4 closing the tray 20. In this arrangement, the
lid is adapted to serve the purpose of the support means after the container has been
opened. For that, the lid consists of a protruding portion 45 which can be separated
from the rest of the lid and then positioned within the cavity of the tray to form
the support means 3 for the frozen food block. The protruding portion 45 of the lid
is, for example, a plate-like part with a peripheral edge extending downwardly from
the plate-like part so as to maintain a predetermined constant spacing between the
frozen block and the bottom of the tray. The protion 45 of the lid will be in a material
transparent to the microwaves for the reasons already previously explained in details.
The tray 20 is also made in accordance with the specificity of the invention as previously
explained. The lid may be attached by any suitable means to the tray, for example,
by thermosealing, adhesion, mechanical connections, or similar attaching means. Preferably,
the portion of the tray 45 is detachable from the rest of the tray by other independent
attaching means.
[0052] In FIG. 14, the container 10 comprises a tray 20 containing the food block and a
lid 4 closing the tray as in the previous example. The lid made of microwave-transparent
material can be separated from the tray 20 and turned up side down to fit into the
container. The lid is shaped so as to form a cavity 46 for receiving the food block.
The food block is so transferred from the tray to the cavity of the lid 4. The tray
provides a firm support for the lid preferably by means of side edges 220 protruding
outwardly from the side wall onto which abut complementary side edges 40 of the lid.
The lid is sized so as to leave a predetermined free space 6 when the lid is properly
fitted within the tray. As the tray comprises reflective side wall 22 that entirely
surrounds the lid when in reversed position in the tray, the microwave radiation can
be shielded laterally and reflected inside the container in the direction of the food
block. The tray 20 is preferably monobloc and made of aluminium-based material whereas
the lid is a relatively rigid or semi-rigid food-acceptable plastic.
[0053] FIG. 15 and 16 show another construction in which the support means 3 comprises an
inflatable support member capable of supporting the food load to a predetermined elevation
with respect to the reflecting bottom of the tray. The support member may be preferably
a supple inflatable bag comprising a series of airtight cells 35 defining interior
channels 36. The channels 36 are connected to allow air to pass from one cell to the
other until the entire bag is properly inflated to a predetermined thickness. The
bag is inflated by means of a valve 37 connected to the channels 36. The bag may be
made of a material such as a resilient plastic or rubber which is transparent to microwave
radiation.
[0054] FIG. 17 and 18 illustrate another variant of construction of the invention. The container
10 comprises an assembly of interchangeable tray members 20a, 20b. In FIG. 17, the
container 10 has a lower member 20b in which the food block 5 is positioned. The lower
member is made of a microwave-transparent material such as plastic or similar. The
lower member is closed by an upper tray member 20a of larger section and made of a
material having microwave-reflecting ability. The upper tray member 20a has a side
wall extending downwardly which engages externally the side wall of the lower tray
member 20b. In this configuration, the container is preferably assembled and properly
sealed to guarantee tamper evidence of the packaging. When reheating of the food block
is desired, the upper member is opened and then reversed to fit with the lower member.
As the reflecting member 20a has a larger section than the microwave-transparent member
20b, it provides a proper shield against the microwave radiation below and partly
on the side of the food block. Support means such as an inner shoulder or small evenly
distributed corrugations (not illustrated) permits to maintain a predetermined elevation
of the food block with respect to the bottom portion of the reflecting member 20a
by limiting the depth of the microwave-transparent member 20b within the reflecting
member 20a.
[0055] While the invention has been described with regards to specific embodiments, it should
be noted that various modifications, adaptations and changes might be made herein
without departing from the spirit of the invention and scope of the appended claims.
For example, the size and shape of the container contemplate numerous possible variations.
For example, the container may serve for heating or thawing non-frozen meals such
as chilled products or shelf stable food products at ambient.
1. A container (10) to be used in microwave oven comprising
(a) a tray (20) comprising a bottom wall (21) and
a continuous side wall (22) attached to said bottom wall (21) which extends upwardly
from the bottom wall (21), said bottom wall and side wall defining an interior cavity
(23), and
(b) support means (3) for providing support to a frozen food block (5), said support
means being capable to be positioned within the cavity (23) to maintain the food block
in an elevated position with respect to the bottom wall so as to form a predetermined
vertical free space (6) between the bottom wall (21) of the tray and the frozen food
while the frozen food is heated, the support means being made of a substantially transparent
material to the microwaves, wherein,
the bottom wall (21) and at least part of the side wall (22) comprise at least
one continuous shielding layer which horizontally and peripherally delimits the free
space (6), underneath the food block, so as to totally reflect the microwave beams
toward the food block.
2. A container according to claim 1, wherein the continuous shielding layer extends upwardly
along the side wall (22) at least beyond the region where the frozen food upper surface
contacts the side wall.
3. A container according to claim 1 or 2, wherein the free space (6) has a vertical length
(L) of at least 2 millimetres.
4. A container according to claim 3, wherein the free space (6) has a vertical length
(L) comprised between 5 to 20 millimetres.
5. A container according to any of the preceding claim, wherein the layer is made of
metallic material.
6. A container according to claim 5, wherein the layer is made of aluminium or aluminium
alloy.
7. A container according to any of the preceding claim wherein the tray (20) is entirely
made of a monolithic material reflecting microwave radiation.
8. A container according to claim 7, wherein the tray (20) is entirely made of aluminium
or aluminium alloy material.
9. A container according to any of claims 1 to 6, wherein the tray comprises a multilayer
arrangement having at least one layer (70) which is a layer reflecting microwaves.
10. A container according to any of the preceding claim wherein the support means (3)
comprises a plate-like portion (30) for receiving the food block and at least one
spacing member (31) between the bottom wall and the plate-like portion.
11. A container according to claim 10, wherein the spacing member (31) is part of the
plate-like portion (30).
12. A container according to claim 10 or 11, wherein the spacing member (31) forms a series
of discrete spacing members (31a, 31b, 31c, 31d) evenly distributed and attached under
the plate-like portion (30).
13. A container according to claim 10, wherein the spacing member (31) is part of the
tray (20).
14. A container according to claim 13, wherein the spacing member (31) forms a peripheral
shoulder (24) of the tray onto which the plate-like portion is positioned.
15. A container according to any of claims 1 to 9, wherein the spacing member (31) is
a plurality of glass or plastic marbles (33) directly contacting the food block.
16. A container according to any of the preceding claims wherein the side wall comprises
a number of substantially angled portions (221, 222, 223, 224); in the vicinity of
which the upper surface of the tray is partially covered by microwave opaque portions
(41, 42, 43, 44) so as to form substantially triangular-shaped microwave trapping
regions.
17. A container according to claim 16, wherein it further comprises a lid (4) having a
main central portion made of a material transparent to microwave and said microwave
opaque portions (41, 42, 43, 44) attached to said main central portion positioned
in each corner of the lid.
18. A container according to any of claims 1 to 8, wherein it further comprises a lid
(4), at least a portion (45) of which being adapted to form the support means after
the container has been opened.
19. A container according to claim 18, wherein the lid (4) is capable to fit the tray
(20) in an up-side-down configuration so as to be capable of receiving the food block
(5) at a predetermined elevation.
20. A container according to any of claims 1 to 8, wherein the support means is collapsible.
21. A container according to claims 20, wherein the support means (3) comprises a supple
inflatable bag comprising a series of air cells (35) defining interior channels (36).
22. A container according to any of claims 1 to 8, wherein the container comprises an
assembly of interchangeable tray members (20a, 20b), a first microwave-transparent
member (20b) in which the food block is positioned and a second reflective-microwave
member (20a) of larger section externally engaging the first member (20b) in a closed
configuration for closing the container, the first member (20b) being capable to fit
at a predetermined elevated depth within the second member (20a) when reversed in
a heating configuration.
23. A container according to any of the preceding claims wherein the cavity (23) has a
size of at least 1 litre.
24. A method of microwave heating food products comprising
positioning the food product (5) in a container (10) while leaving a sufficiently
large uppersurface of the container transparent to microwave energy to allow an amount
of microwave energy to enter the food product,
leaving a predetermined free space (6) under the food product by elevating the food
product in the container,
providing a continuous shielding arrangement of at least a part of the container corresponding
to the external limits demarcating the free space (6) so as to produce a total reflection
of the microwave beams toward the food block.
25. A method according to claim 24, wherein the food product is a frozen food block (5).
26. A method according to claim 24 or 25, wherein the free space (6) has a vertical length
(L) comprised between 5 to 20 millimetres.
27. A method according to claim 25, wherein the free space (6) is about 10 mm for a frozen
food product having a thickness comprised between 28 to 30 mm.